This invention relates to a variable venturi carburetor for an internal combustion engine in an automobile.
In a conventional variable venturi carburetor as shown in FIG. 1, a suction piston 3 is transversely moved with respect to a venturi portion 2a in an air intake passage 2 in response to the load conditions of an engine to vary the venturi portion 2a. A fuel metering needle 4 is fixed at its base portion to the end of the suction piston 3 and is slidably inserted at its tip portion into a main fuel jet 14 for controlling the opening area of a fuel metering portion of the main fuel jet 14 by reciprocating motion of the suction piston 3 to vary the amount of fuel supplied from the main fuel jet 14 through a main nozzle 5 to the air intake passage 2. Reference numeral 6 designates a throttle valve provided in the air intake passage 2. Reference numerals 7 and 8 are a suction chamber formed in a cylindrical portion 1a of the carburetor body 1 and a suction spring disposed in the suction chamber, respectively. An air hole 9 is formed in the vicinity of the inlet of the air intake passage 2, whereby atmospheric air is induced through the hole 9. A suction hole 10 is provided at the right-hand end of the suction piston 3 and is adapted to communicate with the suction chamber 7 and the venturi portion 2a. Reference numerals 11, 12 and 13 designate a float chamber, a float and a fuel pipe, respectively. In the prior art carburetor, especially at idle and low-speed engine operations, since the transverse distance between the suction piston 3 and the main nozzle 5 becomes shorter, a part of fuel supplied through the fuel nozzle 5 fails to flow along with the induced air flowing in the air intake passage 2. As a result, fuel atomization at the venturi portion 2a is disadvantageously reduced over high-speed engine operation. Consequently, as shown in FIGS. 4A and 5A, fuel is apt to adhere to the fuel metering needle 4 and flow down along the inner surface of the air intake passage 2, thus dripping down toward the throttle valve 6. As a result, the air-fuel ratio of the air-fuel mixture to be sucked into the engine is likely to fluctuate, thereby leading to increased fuel consumption as well as to insufficient purification of exhaust gas.